I think Osama's "dirty bomb" and modern nuclear bombs are different things
As I understand it, Osama's "dirty bomb" explodes and spreads radioactivity (i.e.: plutonium dust?) over a large area. It would have a very small blast area and a very large plume of radioactive waste. The big purpose is to terrorize people with the fear of what the radioactivity will do and cause a very expensive cleanup with a very cheap weapon. As far as I know, Osama's "dirty bomb" does utilize a nuclear reaction. It uses a normal chemical explosive as a delivery device to spread radioactivity over a large area.
There are also "dirty bombs" that are nuclear. These are usually found in countries that are new to the nuclear club, i.e.: india, pakistan, etc. They utilize a nuclear reaction to create a very large explosion to destroy things inside the blast zone However, the technology is not cutting edge so the nuclear reaction is not efficient and some of the radio active material is spread with the bomb. They would have a very large blast area and a large nuclear waste plume.
I think the newer nuclear weapons are much "cleaner" as the nuclear reaction is more efficient and utilizes more of the radioactive material.
A neutron bomb has a smaller blast area, kills living things in a relatively large area, but leaves the assets (buildings, vehicles) in the kill zone and outside of the blast area intact. In fact, an army can utilize this material to advance the hostilities, or the area can simply be re colonized when hostilities cease.
The general purpose nuclear bomb's simply destroys everything in the large blast zone.
From a military perspective, simply spreading nuclear material over a large geographical area is not desirable, as the radioactivity is a danger to your forces and makes the area unsuitable to hold after the hostilities cease. It also does nothing for a military strategically, as you have done nothing to reduce the lethal force of the opposing side.
From a terrorists stand point, spreading radioactive material is idea, as it reallocates resources to a very expensive cleanup, makes people afraid to go outside and creates a paranoia where every new freckle or zit becomes suspect.
But like you said, a big nuclear explosion or a fine dust of plutonium falling is very scary.
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Rob
SW Montana's Online Community Modular Model Railroading